16-bit floats and GL_HALF_FLOAT

Question

I'm looking for/writing a C++ implementation of a 16-bit floating point number to use with OpenGL vertex buffers (texture coordinates, normals, etc). Here are my requirements so far:

• Must be 16-bit (obviously).
• Must be able to be uploaded to an OpenGL vertex buffer using GL_HALF_FLOAT.
• Must be able to represent numbers beyond -1.0 - +1.0 (Otherwise I would just use GL_SHORT normalized).
• Must be able to convert to and from a normal 32-bit float.
• Arithmetic operations do not matter - I only care about storage.
• Speed is not a primary concern, but correctness is.

Here's what I have so far for an interface:

class half
{
public:
    half(void) : data(0) {}
    half(const half& h) : data(h.data) {}
    half(const unsigned short& s) : data(s) {}
    half(const float& f) : data(fromFloat(f)) {}
    half(const double& d) : data(fromDouble(d)) {}

    inline operator const float() { return toFloat(data); }
    inline operator const double() { return toDouble(data); }

    inline const half operator=(const float& rhs) { data = fromFloat(rhs); return *this; }
    inline const half operator=(const double& rhs) { data = fromDouble(rhs); return *this; }

private:
    unsigned short data;

    static unsigned short fromFloat(float f);
    static float toFloat(short h);

    inline static unsigned short fromDouble(double d) { return fromFloat((float)d); }
    inline static double toDouble(short h) { return (double)toFloat(h); }
};

std::ostream& operator<<(std::ostream& os, half h) { os << (float)h; }
std::istream& operator>>(std::istream& is, half& h) { float f; is >> f; h = f; }

Ultimately, the real meat of the class lies in the toFloat() and fromFloat() functions, which is what I need help with. I've been able to find quite a few examples of 16-bit float implementations, but none of them mention whether or not they can be uploaded to OpenGL or not.

What are some concerns I should be aware of when uploading a 16-bit float to OpenGL? Is there a half-float implementation that specifically addresses these concerns?

EDIT: By popular demand, here is how my vertex data is generated, uploaded, and rendered.

Here is how the data is defined within the WireCubeEntity class:

VertexHalf vertices[8] = {
        vec3(-1.0f, -1.0f, -1.0f),
        vec3(1.0f, -1.0f, -1.0f),
        vec3(1.0f, 1.0f, -1.0f),
        vec3(-1.0f, 1.0f, -1.0f),
        vec3(-1.0f, -1.0f, 1.0f),
        vec3(1.0f, -1.0f, 1.0f),
        vec3(1.0f, 1.0f, 1.0f),
        vec3(-1.0f, 1.0f, 1.0f)
    };

    unsigned char indices[24] = {
        0, 1,
        1, 2,
        2, 3,
        3, 0,
        4, 5,
        5, 6,
        6, 7,
        7, 4,
        0, 4,
        1, 5,
        2, 6,
        3, 7
    };

    va.load(GL_LINES, VF_BASICHALF, 8, vertices, GL_UNSIGNED_BYTE, 24, indices);

where va is an instance of VertexArray. va.load is defined as:

MappedBuffers VertexArray::load(GLenum primitive, VertexFormat vertexFormat, unsigned int vertexCount, void* vertices,
                                                  GLenum indexFormat, unsigned int indexCount, void* indices)
{
    MappedBuffers ret;

    /* Check for invalid primitive types */
    if (primitive > GL_TRIANGLE_FAN)
    {
        error("in VertexFormat::load():\n");
        errormore("Invalid enum '%i' passed to 'primitive'.\n", primitive);
        return ret;
    }

    /* Clean up existing data */
    clean();

    /* Set up Vertex Array Object */
    glGenVertexArrays(1, &vao);
    bindArray();

    /* Create Vertex Buffer Object */
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, vertexSize(vertexFormat) * vertexCount, vertices, GL_STATIC_DRAW);
    if (!vertices) ret.vmap = glMapBuffer(GL_ARRAY_BUFFER, GL_WRITE_ONLY);

    /* Save variables for later usage */
    prim = primitive;
    vformat = vertexFormat;
    vcount = vertexCount;

    /* If we've been given index data, handle it */
    if (indexSize(indexFormat) != 0)
    {
        glGenBuffers(1, &ibo);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, indexSize(indexFormat) * indexCount, indices, GL_STATIC_DRAW);
        if (!indices) ret.imap = glMapBuffer(GL_ELEMENT_ARRAY_BUFFER, GL_WRITE_ONLY);

        iformat = indexFormat;
        icount = indexCount;
    }

    /* Handle the vertex format */
    switch (vformat)
    {
    case VF_BASIC:
        /* VF_BASIC only has a position - a 3-component float vector */
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
        break;
    case VF_32:
        /* VF_32 has 3 components for position, 2 for texture coordinates, and 3 for a normal.
        Position is at offset 0, TextureCoordinate is at offset 12, and Normal is at offset 20 */
        glEnableVertexAttribArray(0);
        glEnableVertexAttribArray(1);
        glEnableVertexAttribArray(2);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, vertexSize(VF_32), (void*)0);
        glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, vertexSize(VF_32), (void*)12);
        glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, vertexSize(VF_32), (void*)20);
        break;
    case VF_BASICHALF:
        /* VF_BASICHALF is very similar to VF_BASIC, except using half-floats instead of floats. */
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_HALF_FLOAT, GL_FALSE, 0, (void*)0);
        break;
    case VF_WITHTANGENTS:
        /* VF_WITHTANGENTS is similar to VF_32, but with additional components for a Tangent. */
        /* Tangent is at offset 32 */
        glEnableVertexAttribArray(0);
        glEnableVertexAttribArray(1);
        glEnableVertexAttribArray(2);
        glEnableVertexAttribArray(3);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, vertexSize(VF_WITHTANGENTS), (void*)0);
        glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, vertexSize(VF_WITHTANGENTS), (void*)12);
        glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, vertexSize(VF_WITHTANGENTS), (void*)20);
        glVertexAttribPointer(3, 3, GL_FLOAT, GL_FALSE, vertexSize(VF_WITHTANGENTS), (void*)32);
        break;
    default:
        error("In VertexFormat::load():\n");
        errormore("Invalid enum '%i' passed to vertexFormat.\n", (int)vformat);
        clean();
        return MappedBuffers();
    }

    /* Unbind the vertex array */
    unbindArray();

    if (vertices) ready = true;

    return ret;
}

I'ts a pretty heavy function, I know. MappedBuffers is simply a struct that contains 2 pointers so that if I pass NULL data into VertexArray::load(), I can use the pointers to load the data directly from file into buffers (possibly from another thread). vertexSize is a function that returns the sizeof() of whichever vertex format I pass in, or 0 for an invalid format.

The VertexHalf struct is:

struct VertexHalf
{
    VertexHalf(void) {}
    VertexHalf(vec3 _pos) :x(_pos.x), y(_pos.y), z(_pos.z) {}
    VertexHalf(float _x, float _y, float _z) : x(_x), y(_y), z(_z) {}

    half x, y, z, padding;
};

And finally the data is rendered using the VertexArray we loaded earlier:

void VertexArray::draw(void)
{
    if (ready == false)
        return;

    /* Bind our vertex array */
    bindArray();

    /* Draw it's contents */
    if (ibo == 0)
        glDrawArrays(prim, 0, vcount);
    else
        glDrawElements(prim, icount, iformat, NULL);

    unbindArray();
}

Answer 1

Edit: The most obvious error appears in your VertexHalf structure. You have an element of padding. Yet when you specify your glVertexAttribPointer you specify a 0 in the stride which indicates it is tightly packed. So you can either change VertexHalf to remove the padding or change your glVertexAttribPointer to have a stride of 8 bytes.

I use the following class with DirectX for float16 support and it works perfectly.

Float16.h:

#ifndef THE__FLOAT_16_H_
#define THE__FLOAT_16_H_

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

extern short FloatToFloat16( float value );
extern float Float16ToFloat( short value );

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

class Float16
{
protected:
    short mValue;
public:
    Float16();
    Float16( float value );
    Float16( const Float16& value );

    operator float();
    operator float() const;

    friend Float16 operator + ( const Float16& val1, const Float16& val2 );
    friend Float16 operator - ( const Float16& val1, const Float16& val2 );
    friend Float16 operator * ( const Float16& val1, const Float16& val2 );
    friend Float16 operator / ( const Float16& val1, const Float16& val2 );

    Float16& operator =( const Float16& val );
    Float16& operator +=( const Float16& val );
    Float16& operator -=( const Float16& val );
    Float16& operator *=( const Float16& val );
    Float16& operator /=( const Float16& val );
    Float16& operator -();
};

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16::Float16()
{
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16::Float16( float value )
{
    mValue  = FloatToFloat16( value );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16::Float16( const Float16 &value )
{
    mValue  = value.mValue;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16::operator float()
{
    return Float16ToFloat( mValue );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16::operator float() const
{
    return Float16ToFloat( mValue );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator =( const Float16& val )
{
    mValue  = val.mValue;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator +=( const Float16& val )
{
    *this   = *this + val;
    return *this;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator -=( const Float16& val )
{
    *this   = *this - val;
    return *this;

}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator *=( const Float16& val )
{
    *this   = *this * val;
    return *this;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator /=( const Float16& val )
{
    *this   = *this / val;
    return *this;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16& Float16::operator -()
{
    *this   = Float16( -(float)*this );
    return *this;
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/
/*+----+                                 Friends                                       +----+*/
/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16 operator + ( const Float16& val1, const Float16& val2 )
{
    return Float16( (float)val1 + (float)val2 );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16 operator - ( const Float16& val1, const Float16& val2 )
{
    return Float16( (float)val1 - (float)val2 );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16 operator * ( const Float16& val1, const Float16& val2 )
{
    return Float16( (float)val1 * (float)val2 );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

inline Float16 operator / ( const Float16& val1, const Float16& val2 )
{
    return Float16( (float)val1 / (float)val2 );
}

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/


#endif

Float16.cpp:

#include "Types/Float16.h"

//#include <d3dx9.h>

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

short FloatToFloat16( float value )
{
    short   fltInt16;
    int     fltInt32;
    memcpy( &fltInt32, &value, sizeof( float ) );
    fltInt16    =  ((fltInt32 & 0x7fffffff) >> 13) - (0x38000000 >> 13);
    fltInt16    |= ((fltInt32 & 0x80000000) >> 16);

    return fltInt16;
}

/+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+/

float Float16ToFloat( short fltInt16 )
{
    int fltInt32    =  ((fltInt16 & 0x8000) << 16);
    fltInt32        |= ((fltInt16 & 0x7fff) << 13) + 0x38000000;

    float fRet;
    memcpy( &fRet, &fltInt32, sizeof( float ) );
    return fRet;
 }

/*+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+*/

Answer 2

The GLM library supports half-float types. The prefix used is 'h' so where glm::vec3 is a 3 element vector of floating points values, glm::hvec3 is a 3 element vector of half-floats.